The invention relates to a hydraulic braking system operated by an external force, for vehicles, in particular motor vehicles, with at least two axles or axle lines, with                a pressureless or low-pressure-side hydraulic reservoir,        at least one primary pressure source which is fed by external energy and is connected to the hydraulic reservoir for the supply of hydraulic medium,        an auxiliary pressure source which is independent of the primary pressure source and can be actuated by foot or hand and which is connected to the hydraulic reservoir for the supply of a hydraulic medium and the delivery side of which communicates with the hydraulic reservoir in the non-actuated state of the auxiliary pressure source,        displacer assemblies, which are assigned in each case to a wheel brake separately for the actuation of the latter,        normally closed controllable inlet valves which are in each case assigned separately to a displacer assembly for the controllable connection of the latter to the primary pressure source or to one of the primary pressure sources,        normally open connecting valves which are assigned separately to each displacer assembly and parallel to the respective inlet valve and which are connected to one another, axle by axle, on their side facing away from the assigned displacer assembly,        a normally closed outlet valve which is arranged between the hydraulic reservoir and the interconnected sides of the connecting valves of a first axle,        a normally open blocking valve which is arranged between the delivery side of the auxiliary pressure source and the interconnected sides of the connecting valves of a first axle.        
Such a braking system operated by an external force is the subject of U.S. Pat. No. 5,988,768.
In this braking system operated by an external force, moreover, the pedal actuating the auxiliary pressure source actuates a desired-value generator of a regulating arrangement which shuts off the separating valve and opens the outlet valve when the braking system operated by an external force is functioning normally. Otherwise, depending on the respective desired value, the control arrangement controls the inlet and connecting valves of the displacer assembly of the wheel brakes as a function of the sensor-detected hydraulic actual pressure of the displacer assemblies or wheel brake cylinders.
Should the pressure of the primary pressure source faultily fall, the separating valve and the connecting valves automatically fall back to their normally open state, while the inlet valves and the outlet valve change over to their normally closed state. The brake cylinders of the axle assigned to the outlet valve can consequently be actuated by means of the auxiliary pressure source.
Similar braking systems are the subject of DE 43 43 386 A1 and of U.S. Pat. No. 5,866,822.
A braking system operated by an external force, known from DE 196 36 432 A1, possesses a pressureless hydraulic reservoir, a primary pressure source, which consists of an electric motor driven pump connected on the suction side to the hydraulic reservoir and of a pressure accumulator which is arranged on the delivery side of the pump and is protected against discharge by the pump by means of a non-return valve and which is recharged by the pump as a function of pressure, and a pedal-actuated auxiliary pressure source which is independent of said primary pressure source and is designed in the manner of a conventional brake master cylinder arrangement and the delivery side of which communicates with the hydraulic reservoir in the non-actuated state of the auxiliary pressure source. Furthermore, each vehicle wheel is provided with a wheel brake with a displacer assembly or wheel brake cylinder for actuating the wheel brake. These displacer assemblies can be connected in each case via normally closed controllable inlet valves to the delivery side of the primary pressure source or to the pressure accumulator and, for pressure relief, can be connected to the hydraulic reservoir via normally closed controllable outlet valves assigned separately to each wheel brake. Moreover, the wheel brake cylinders of the wheels of each axle are connected to one another via a connecting line which can be separated by means of a normally open stop valve and which communicates on one side of the stop valve with the auxiliary pressure source via a normally open valve.
In this braking system operated by an external force, moreover, the pedal of the auxiliary pressure source actuates a desired-value generator of a regulating arrangement which, when the braking system operated by an external force is functioning normally, shuts off the separating valves and, depending on the respective desired value, controls the inlet and outlet valves of the displacer assemblies of the wheel brakes as a function of the sensor-detected hydraulic actual pressure of the wheel brake cylinders. In this case, on the one hand, wheel-by-wheel control of the pressure of the displacer assemblies is possible; for this purpose, the shut-off valves between the displacer assemblies of an axle are closed and the pressures of the displacer assemblies are set by means of the inlet and outlet valves assigned to them. On the other hand, axle-by-axle pressure control of the displacer assemblies may also take place in that the connecting valves between the displacer assemblies of an axle are opened. In this case, to control the pressure of the displacer assemblies communicating with one another, it is sufficient merely to actuate one of the inlet valves assigned to these displacer assemblies and one of the assigned outlet valves, while the remaining inlet and outlet valves remain closed.
All the inlet and outlet valves of this braking system operated by an external force are designed as identical differential-pressure solenoid valves, so that the maximum obtainable hydraulic pressure remains limited even when the pump charging the pressure accumulator faultily operates continuously, although the pressure accumulator has already reached its charging pressure.
Should the pressure of the pressure accumulator faultily fall, the separating and connecting valves automatically fall back into their normally open state, while the inlet and outlet valves change over to their normally closed state. The brake cylinders and, correspondingly, the wheel brakes can then be actuated by means of the auxiliary pressure source.
DE 196 22 726 A1 discloses a hydraulic vehicle braking system, in which each vehicle wheel is assigned a wheel brake with a displacer assembly or wheel brake cylinder for actuating said wheel brake. These displacer assemblies can be connected in each case via normally opened controllable inlet valves to the brake master cylinder or the delivery side of a return pump and, for pressure relief, to the suction side of the return pump via normally closed controllable outlet valves assigned separately to each wheel brake. A normally open controllable changeover valve is arranged between the master brake cylinder and the circuit between the displacer assemblies and return pump. A normally closed controllable intake valve is arranged, parallel to this changeover valve, between the brake master cylinder and the suction side of the return pump.
It is the object of the present invention to provide a braking system operated by an external force, which, on the one hand, allows multiply variable control and, on the other hand, has a particularly high safety reserve.